Not Applicable
Not Applicable
There are various ways of sensing the rotation of an induction motor. Tachogenerators, resolvers and Hall-effect sensors are some of the conventional sensing devices. But all such sensing methods necessitate the use of additional hardware on the induction-motor frame itself. In many applications, where there are physical or cost constraints, it may not be possible to use additional sensing devices on the motor frame. When an induction motor is being driven by using a variable-speed drive, it is possible to define or estimate the angular movement of the rotor. But when the power to the drive is turned off, and the induction motor keeps rotating either due to its rotor""s moment of inertia or due to some external mechanical force, it is not possible to sense the angular movement of the rotor of the induction motor without the use of an extra device coupled with the frame of the induction motor.
In many applications, it becomes essential to be able to detect the angular movement of the rotor of the induction motor after electrical power driving the motor has been switched off. Like in the case of the high-rpm induction motor driving high-speed grinding wheels resting on oil-fed hydrostatic bearings. At the time of sudden power outage, if the grinding wheels are rotating, it is essential to continue oil supply to the hydrostatic bearing. As there are physical constraints to introducing an additional conventional sensor in the above-mentioned example, a solution is needed to sense the angular movement of the driving induction motor""s rotor. There are low-cost areas of application, like electrical saws and industrial blowers; where the easy sensing of rotation of the driving induction motor after electrical power to the motor is withdrawn can provide a safety interlock without any alteration in the mechanical construction of such equipment.
The rotor of a common induction motor is in the form of multiple shorted secondary windings of a transformer. The electrical conductor is made either of aluminum or copper; the magnetic circuit comprises of silicon-steel stampings stacked together. When power is switched off to an induction motor, the residual magnetism of the silicon-steel stampings forms multiple poles on the angular face of the rotor. These multiple magnetic poles induce minute electrical current in the stator winding just like as it happens in a small alternator. The frequency of the output voltage generated in this manner is directly proportional to the angular velocity of the rotor of the induction motor. Contactors and relays are arranged in a manner so that this sensor voltage could be directed to an amplifier and/or counter to be able to draw inference from the sensor signal.
The absence of any external device on the induction motor makes this invention particularly easy to implement. Many a time it is physically impossible to accommodate any piece of hardware in and around an induction motor, in these circumstances the present invention could be of particular use. In addition, in applications where the motor is not powered and is rotating due to some other mechanical linkages, this invention could be used to detect that movement without putting any additional device on the motorxe2x80x94thus saving both cost and space.
This invention successfully solves the problem of detecting during a power outage the rotation of the rotor of the spindle-driving induction motor in a grinding machine with hydrostatic bearings supporting the spindle, which holds big grinding wheels. The impossibility of fitting an extra sensor in the extremely harsh conditions near the grinding wheels means having a sensor on the spindle-driving induction motor, which is physically very difficult. Any alteration in the mechanical mounting of the spindle-driving induction motor could disturb the mechanical alignment, which is absolutely necessary for a satisfactory operation of the grinding machine. This invention obviates the need to even touch the mechanical parts of the grinding machine and solves the problem by using the spindle-driving induction motor as a sensor to detect its own rotation during a power outage to be able to give command to a battery-powered inverter to commence electrical supply to the oil-pump motor supplying oil to the very expensive and critical hydrostatic bearings supporting the spindle.